Group III-V semiconductor transistors, such as high electron mobility transistors (HEMTs), require smaller gate contact lengths (Lg) (less than 40 nm) and low gate contact resistance (Rg) for high frequency operation. However the smaller gate contact size normally associates with higher gate resistance (Rg) due to the smaller physical dimensions of gate contact length. Historically, fabrication of a T-shaped gate contact (T-gate contact) approach has been implemented in the industry for group III to V semiconductor transistors. The T-shaped gate contact employs a small gate contact portion and a larger top contact portion, referred to as a wing. The purpose of the larger top contact portion (wing) is to increase the cross sectional area of gate (to lower the Rg) while maintaining the smaller gate length (lower Lg). However, when the T-gate is scaled down to smaller gate contact sizes (less than or equal to 40 nm), the wing contact portion size scales down proportionally and has smaller dimensions.
Some techniques have been employed to form gate contacts with smaller gate contact portion size with larger wing contact portions. One of the techniques employed to form gate contacts with smaller gate contact portion sizes with larger wing contact portions involves using two separate exposure/development steps and two separate metalization steps. The gate contact is formed in the first resist exposure, development, and metallization processing step, and the wing contact portion in the second resist exposure, development, and metallization processing step. This process would allow the wing to be sized to many selective wing sizes. However the disadvantages are that the registration of the wing contact portion to the gate contact portion can be misaligned. Furthermore, the increased number of processing steps to achieve the results will lower the total gate yield when dealing with gate sizes of 40 nm or smaller. The number of processing steps will also effectively increase the length of processing time and cost.
Another technique is to use a two step resist exposure process of exposing and developing a resist of a wing contact portion first and then exposing and developing a resist of a gate contact portion. However there can be misalignment of wing contact portion to gate contact portion registration, and the gate size control/uniformity can degrade by wing dose contribution to the gate dose.
A third technique is to use multiple layers of resist, and use a set of developers. Each developer specifically develops each resist layer individually. This allows the wing size to be developed for longer time resulting in a larger wing. However, each of the resist layers needs to be developed completely, so that the next developer can develop the next layer correctly. Any resist not completely developed away will prevent the next developer from opening the resist for the next layer. Additionally, the different resists can form an inter-mixed layer which can be difficult to develop out completely by any of the developers.